This invention relates to coatings on substrates for protecting the substrates from undesirable environmental reactions.
This invention relates to methods of providing a surface of a carbon based material, metal, metal alloy, or ceramic substrate with a crack limiting coating which prevents developing cracks from extending through the coating both under isothermal conditions and when the temperature is changed. The through-crack preventing coating prevents oxygen, or other reactive agents ambient to the coating, from penetrating through the coating and contacting the substrate.
This invention relates particularly to introducing fine particulate (such as silicon carbide, hafnium oxide, zirconium oxide, aluminum oxide, silicon oxide, hafnium nitride, silicon nitride, hafnium diboride, or similar compounds) into the coating.
Carbon--carbon composites and graphite undergo catastrophic oxidation at elevated temperatures when oxygen is available to carbon based substrate. Other substrate materials can also require protection from oxygen or other reactive agents ambient to a protective coating.
Oxidation prevention requires complete encapsulation with a protective material. The coating should have no continuous cracks or porosity, in order to achieve such protection.
Carbon based substrates require protection for many aerospace applications.
Many conventional coatings, including those deposited by CVD methods, are prone to cracking because of a large difference in coefficient of thermal expansion (CTE) between carbon and most of the potential coating materials. The large difference in CTE leads to tensile stresses in cool down and associated cracking of the coatings.
Some coatings suffer from poor adhesion to the substrate, also leading to coating failure.
For appropriate functioning, coatings should be mechanically and chemically stable under extreme thermal and oxidative environments, should provide good adhesion to the substrate, and should offer good thermal shock resistance, low oxygen and carbon permeability, and should offer low reactivity with the substrate at the operating temperatures.
Coatings should also, desirably, possess minimal mismatch in CTE with the substrate.
Ceramic compounds, such as oxides, carbides, silicides, nitrides, and borides are often employed as coatings. The ceramic compounds provide good performance at high temperatures, but the ceramic compounds are brittle and have a large CTE mismatch with carbon based substrates. The ceramic compounds therefore tend easily to crack.
A number of methods have been used in preparing protective coatings.
These methods include chemical vapor deposition (CVD), physical vapor deposition (PVD), pack cementation and a combination of pack cementation and CVD.
Coating materials have been prepared in layered structures in attempts to minimize the effects of CTE mismatch. Layering has been aimed at grading the CTE as well as improving the compatibility between layers and the substrate.
Another approach to coatings has involved fused slurry processing. The fused slurry processing has been applied to the processing of Al--Si and Ni--Si slurry coatings on C--C composites. Coatings using these transition metals were not totally protective in the 1,200.degree. C. cyclic oxidation test because of incomplete coverage, volatilization losses or localized oxidation at the cracks in the coating.
The simplicity and attractive economics of the slurry coating process (as compared to the more expensive CVD and PVD processes) is, however, a strong motivation for using that slurry coating process, if it is possible to obtain successful results.